During the past several years, the popularity and viability of fuel cells for producing both large and small amounts of electricity has increased significantly. Fuel cells conduct an electrochemical reaction with reactants such as hydrogen and oxygen to produce electricity and heat. Fuel cells are similar to batteries except they can be “recharged” while providing power. In addition, fuel cells are much cleaner than other sources of power, such as devices that combust hydrocarbons.
Fuel cells provide a DC (direct current) voltage that may be used to power motors, lights, computers, or any number of electrical appliances. A typical fuel cell includes an electrolyte disposed between an anode and a cathode. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte used into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC).
While all fuel cells have some desirable features, solid oxide fuel cells (SOFC) have a number of distinct advantages over other fuel cell types. Some advantages of SOFCs include reduced problems with electrolyte management, increased efficiencies over other fuel cell types (SOFCs are up to 60% efficient), higher tolerance to fuel impurities, and the possible use of internal reforming or direct utilization of hydrocarbon fuels.
Most SOFCs include an electrolyte made of a solid-state material such as a fast oxygen ion conducting ceramic surrounded on each side by an electrode: an anode on one side, and a cathode on the other. In order to provide adequate ionic conductivity in the electrolyte, SOFCs typically operate in the 500 to 1000° C. range. An oxidant such as air is fed to the cathode, which supplies oxygen ions to the electrolyte. Similarly, a fuel such as hydrogen is fed to the anode where it is transported to the electrolyte to react with the oxygen ions. This reaction produces electrons, which are then introduced into an external circuit as useful power.
In order to produce a useable amount of power and to increase efficiency, SOFC fuel cells are typically stacked on top of one another to form an SOFC stack. In many such designs, a fuel manifold is sealed around the perimeter of the stack to assure that the fuel and air are separated and directed to their proper locations respectively. Sealing around the entire perimeter requires a long seal length between dissimilar materials.
The difference in material composition between the metal fuel manifolds and the ceramic SOFCs results in different coefficients of thermal expansions. Throughout the operation of an SOFC, a cell is often cycled between room temperature and a full operating temperature a number of times. This thermal cycle causes the housing materials to contract and expand according to their varying coefficients of thermal expansion. This expansion and contraction introduces thermal stresses that may be transferred through the seals and other structural components directly to the ceramic cell. These thermal stresses effectively reduce the service life of an SOFC by compromising the seals or breaking the structurally brittle ceramic cells.
Moreover, typical fuel cell stack arrangements have low volumetric power density due to geometric limitations. Volumetric power density is a measure of the power produced by a fuel cell in relation to its volume. Low volumetric power density may undermine the broad applicability of fuel cell systems due to size considerations.